The macroscopic material world is formed by millions of crystals; every crystal at a microscopic level is formed by a cell having some symmetry for example hexagonal, trigonal, cubic etc, and having a specified quantity of atoms therein whose positions are defined. In order to understand the properties of matter is essential to know the crystal structure at atomic level; this requires atomic resolution structural studies by means of X-Rays, neutron or electron diffraction.
X-Ray diffraction is a technique which provides an average structure, usually of tens of thousands of particles. Newly synthesised materials are often in powder form and often present poor crystallinity for precise X-Ray structure determination.
Electron diffraction (ED) is much more sensitive than X-Ray diffraction, providing about a ten thousand fold enhancement in signal. Electron diffraction structure analysis of materials originally developed in the early 50's when diffraction intensities obtained from electron diffraction cameras were recorded on photographic films. Such films are good for reproducing contrast detail in transmission electron microscope (TEM) imaging, for example, however, they suffer from lack of dynamic range for recording intensity measurements of ED patterns. Therefore, they are not adequate for quantitative and precise structure analysis. Despite the limitations, however, many different structures of organic and inorganic substances have been determined up to now using ED and photographic films, albeit with low resolution. The studies contributed significantly to the fields of crystallography and crystal chemistry of solids.
A means for precise atomic structure analysis is linked to the increasing of reliability and precision in measuring ED intensity data. One important feature for accurate measurement of intensities in an ED pattern is ability of capture the high dynamic range of spot intensities, which can range between 1 and 106 for the weakest and most intense spots respectively.
Another important issue when measuring ED data for resolving crystal structures is the speed at which intensities are measured. Radiation damage to most samples usually occurs for exposures lasting longer than a minute. Organic compounds are especially sensitive to radiation damage. Since the damage is irreversible and renders the crystal unusable for further diffraction studies, and crystals can take many weeks or months to prepare, any technique which reduces the exposure of the crystal to the beam would be advantageous.
In order to resolve successfully a structure from ED data, the intensities of the spots in an ED pattern need to be determined accurately with minimum beam damage to the sample. In practice, for structure analysis using ED, especially where light elements such as oxygen and hydrogen are detected in the cell, the precise measurement of about 50-100 reflections is sufficient to have an accurate picture of the crystal structure. This contrasts with X-Ray diffraction where several hundreds of reflections need to be measured.
For crystal structure determinations, which includes the determination of electrostatic potential and analysis of chemical bonding of crystals, the precision of measuring ED intensities should be high and preferably the same for all reflections. This is true also for the weakest intensities (i.e. those resulting from large diffraction angles), since they have a significant influence on the accurate determination of the light atoms in the structure.
Up to now analysis by ED has been performed, in most cases, using devices of the art whose primary purpose is to record electron diffraction patterns. These devices are known as electron diffraction cameras (EDC), and are characterised in that they do not have imaging lenses, they work at low-intermediate voltages (>100 kV) and have a beam size of the order of few microns. To obtain precise, quantitative analysis using an EDC requires the implementation of a pattern scanning system in front of a fixed counter mounted on the EDC (Avilov et al, J. Appl. Cryst. (1999), 32, 1033-1038)). With this technique the location of light atoms, such as hydrogen may be determined in the studied crystal structures, even when heavier atoms such as iron or aluminium are present. The electrostatic potential distribution and the electron density may be reconstructed in some cases. However, the scanning system has a lot of limitations. For example, it takes several hours to record with precision the reflections of an ED pattern. Therefore, only very few beam resistant samples can be precisely measured with this technique. Moreover, this type of scanning system is interfaced with EDCs whose beam size is generally too wide (micrometer range) for obtaining atom resolution detail.
ED patterns of crystals may also be measured using a transmission electron microscope (TEM). Structure analysis of crystals by ED in a TEM presents a lot of advantages over conventional X-Ray or neutron diffraction: the size of studied crystallites in TEM can be very small (even tens of Ångstroms), therefore individual phases in powders (nm size) can be examined.
Typically a TEM is interfaced to a slow scan charged-coupled device (CCD) camera or imaging plate for recording the ED patterns. However, problems with using a CCD camera or imaging plate for detection exist. For example, the dynamic range of the CCD camera is limited which leads to errors in measuring very intense or very weak intensities. Furthermore, continuous exposure of a CCD camera to high diffracted ED beam intensities can damage the CCD detector. Even, if in the near future those problems could be partially resolved in some commercial CCD cameras, the main problem of precision measurement (up to 1%) of all ED intensities—and specially the weakest ones—will remain. For instance, accuracy in intensity estimation for high angle (>5 Ångstroms−1) reflections is very poor (up to 25% accuracy). Imaging plates for recording ED in a TEM can be constructed with the same area as the photographic films, and may be used together with photographic film in a TEM. Exposed plates may read with a laser beam; this system is claimed to have dynamic range over 100 times more than that of a commercial CCD camera. As with CCD cameras, the main problem of precision measurement (up to 1%) of all ED intensities—and specially the weakest ones—cannot be resolved with this technique.
The present invention relates to a method for obtaining an electron diffraction pattern from any conventional transmission electron microscope (TEM) which overcomes the problems of dynamic range, speed at which intensities are measured and sample deterioration in the beam, accounting for beam instability from which method and devices of the prior art all suffer. The invention is suitable for measuring with high precision electron diffraction intensities, correct them from dynamical contributions, and perform calculations to derive crystal structures at atomic resolution from nanometer size areas, or in the range afforded by the beam size the TEM.